The buzzing problem of Z333…

The issue arises during the standby of the speaker system. More exactly, in order to conserve power and prevent operation when not in use, the Logitech Z333 enters a low power consumption state after about 30 minutes. This is a very good feature but not all the way in this case.

This feature is achieved with a circuit that measures the input gain on the line-in (both RCA an 3,5 mm). Basically if no (significant) input voltage is detected the speaker system automatically enters in a soft stand-by mode until is wakened by a (significant) voltage on the input line.

For the Z333 to wake-up automatically from standby it is important that the output device (audio source) connected the speakers to produce a significant audio signal (voltage) thus it is recommended that the volume level of the audio source device is set above 50%. Recommended input volume level is between 60% and 90%.

The output volume of the Z333 itself can be set lower as it does not make a difference for this. The recommended value is 25% overall volume (main knob) and about 50% subwoofer volume (bass knob).

The Logitech Z333 can also be waken manually from standby by operating the ON/OFF switch on the volume knob or by simply reconnecting the speakers to the mains.

When the speakers are entering the low power mode (standby) the power demand from the internal Switching Mode Power Supply Unit (PSU) is drastically reduced, thus it could be expected that some components from the PSU board start producing a slight buzzing sound.

Unfortunately in this case, any buzzing sound gets amplified because the PSU board of the Z333 is located inside the resonant chamber of the subwoofer unit. Any faint noise coming from the board is gaining echo inside the subwoofer enclosure and can easily be heard especially at night when is silence as you can hear in the following video (audio volume is lifted so you can hear the buzzing):

After some PSU board analysis of Z333 (under power, no load) the buzzing noise origin appears to be one high voltage capacitor on the power circuit for the main switching transistor.

This is a blue bypass capacitor that resonates at audible range close to mains frequency at about 50/60 Hz when the switching frequency of the main switching transistor is low as there very little power demand from the PSU.

This Logitech Z333 speaker system was brand new and the buzzing sound was produced since its first stand-by. The culprit for this problem can be one of the following:

Poor PSU board circuit design not developed considering a second, low power demand mode of operation

Incomplete quality testing both from PSU board manufacturer and Logitech

The 103M10KV is a high-voltage 10 nF ceramic capacitor in paralel with the start-up resistor (R2) for the main switching transistor. The role of this bypass capacitor is to provide a smoother DC power for the main switching transistor – winch it does. Basically the ceramic capacitor was only producing noise at low switching frequency if the PSU was not loaded or with very little load.

The original capacitor (in the image above) may have had some sort of defect or at the best it was not designed with low frequency currents in mind. First time after changing the existing bypass capacitor with another one (the same values) the noise was about the same. Then after changing it once more with yet another capacitor taken from another dead PSU the noise was finally gone.

This is telling us that the issue might be either a slip in the circuit design or improper quality capacitor for the job or a combination of both. Assembling back the subwoofer enclosure is shown in the following pictures: